Japanese Patent Application No. 2001-81744, filed on Mar. 21, 2001, and Japanese Patent Application No. 2001-260427, filed on Aug. 29, 2001, are herein incorporated by reference in their entirety.
The present invention relates to an electrode material for an electric double layer capacitor and an electric double layer capacitor using the electrode material.
An electric double layer capacitor has been used for a backup power supply for a personal computer memory, an auxiliary or backup for a secondary battery, for a backup power supply for an electric vehicle battery or a fuel cell vehicle battery, for a power supply for hybrids, or the like.
When a voltage is applied between two electrodes, ions in an electrolyte solution are absorbed onto surfaces of polarizable electrodes and an electric double layer is formed in the interface between the electrodes and electrolyte solution. The electric double layer capacitor stores charges in this electric double layer (FIG. 16).
This electrolytic double layer capacitor has a large capacitance for storing electricity, about one million times that of a conventional capacitor.
In a conventional electric double layer capacitor, activated carbon is used for the electrodes. Since activated carbon particles have a large number of pores and a large specific surface area, activated carbon is suitable for the electrodes.
The present invention may provide an electric double layer capacitor enabling a further increase in the capacitance by using a carbon fiber having a larger specific surface area, and an electrode material for such electric double layer capacitor.
According to a first aspect of the present invention, there is provided an electrode material for an electric double layer capacitor comprising a coaxial stacking morphology of truncated conical tubular graphene layers, wherein each of the truncated conical tubular graphene layers includes a hexagonal carbon layer.
In other words, this carbon fiber has a cup-stacked structure or a lampshade-stacked structure in which a number of hexagonal carbon layers in the shape of a bottomless cup are stacked. The coaxial stacking morphology of the truncated conical tubular graphene layers may have a shape of a hollow core with no bridge. According to such a structure, each of the truncated conical tubular graphene layers has a large ring end at one end and a small ring end at the other end in an axial direction, wherein edges of the hexagonal carbon layers are exposed at the large ring ends of the outer surface and the small ring ends of the inner surface. In other words, the edges of the tilted hexagonal carbon layers having a herring-bone structure are exposed in layers.
In an ordinary carbon fiber with a herring-bone structure, a number of hexagonal carbon layers in the shape of a cup having a bottom are stacked. However, the carbon fiber according to the first aspect of the present invention has a hollow structure with no bridge and has a length ranging from several tens of nanometers to several tens of microns.
If the coaxial stacking morphology of the truncated conical tubular graphene layers is vapor grown, a wide area of an outer surface or an inner surface may be covered with a deposited film of an excess amount of pyrolytic carbons. However, at least part of edges of the hexagonal carbon layers may be exposed at the large ring ends on the outer surface side or at the small ring ends on the inner surface side.
The edges of the hexagonal carbon layers exposed on the outer surface or the inner surface of the carbon fiber have an extremely high degree of activity, exhibit good affinity to various types of materials, and excel in adhesion to base materials such as resins. Therefore, a composite excelling in tensile strength and compressive strength can be obtained.
In this carbon fiber, part or all of the deposited films formed over the outer surface or the inner surface during the vapor growth process of the carbon fiber may be removed by a treatment to be performed later. It is because the deposited layers are formed of an excess amount of insufficiently crystallized amorphous carbon, and the surfaces of the deposited layers are inactive.
In the carbon fiber according to the first aspect of the present invention, an outer surface of the carbon fiber may be formed of the large ring ends stacked in the axial direction; and exposed part of the edges of the hexagonal carbon layers may have an area equal to or more than 2% of an area of the outer surface, and preferably 7% of an area of the outer surface.
The positions of the large ring ends forming the outer surface may be irregular, and the outer surface may have minute irregularity at the level of atoms.
Similarly, an inner surface of the carbon fiber may be formed of the small ring ends stacked in the axial direction; and positions of the small ring ends forming the inner surface may be irregular, and the inner surface may have minute irregularity at the level of atoms.
The exposed edges of the hexagonal carbon layers have a high degree of activity and can be modified with functional groups such as xe2x80x94COOH, xe2x80x94CHO, or xe2x80x94OH. This enables the carbon fiber to be used as the electrode material.
Since positions of edges of the hexagonal carbon layers exposed on the surfaces of the carbon fiber are irregular, the surfaces have minute irregularity at the level of atoms. Therefore, the specific surface area of the carbon fiber is extremely large, enabling a large-capacitance electric double layer capacitor.
A carbon fiber in which at least part of gaps between the hexagonal carbon layers is larger than the gaps between the hexagonal carbon layers at the time of vapor growth, may be used as the electrode material. In this case, the bulk density of the electrode material may be 0.010 g/cm3 or less. A carbon fiber in which one to several tens of thousands of the hexagonal carbon layers are stacked may also be used.
In the electrode material for an electric double layer capacitor comprising a carbon fiber as an essential material according to a second aspect of the present invention, the carbon fiber has a coaxial stacking morphology of truncated conical tubular graphene layers, and the coaxial stacking morphology of the truncated conical tubular graphene layers includes one to several hundreds of stacked hexagonal carbon layers. Also in this case, at least part of the cyclic edges of the hexagonal carbon layers are exposed on the outer surface or inner surface of the carbon fiber.
According to a third aspect of the present invention, there is provided an electric double layer capacitor in which the above-described electrode material is used for an electrode.